If you were following Autodesk 3ds Max 2012 development process, you've already may know that built-in Havok Reactor physics simulation engine was replaced with a new MassFX unified simulation, using PhysX SDK integration.

However, since only mRigids solver (rigid body and joints dynamics) is available so far, one may say that Reactor was omitted too early, since not all the functionality (cloth, joint motors, forcefields, byoancy) is present in MassFX currently.

Generally, MassFX is based on modified NVIDIA PhysX plug-in for 3ds Max (likely latest 2.60), and thus, resembles its basic functionality and interface, but several differences can be spotted:

Simulation in viewports runs much faster and smoother, thanks to new Nitrous graphics core. Performance Viewer tool, that can be found in PhysX plug-in 2.60, was removed.